1. Field of the Invention
This invention relates to glass bodies (e.g., optical fibers, planar waveguides, glass soot) and, more particularly, to apparatus and methods for fabricating rare-earth (RE)-doped optical fibers.
2. Discussion of the Related Art
In the standard MCVD technique for fabricating optical fibers, preforms are made by flowing relatively high vapor pressure (HVP) precursor gasses (e.g., SiCl4, GeCl4 and POCl3) and oxygen down a rotating silica substrate tube. An external hydrogen-oxygen torch slowly traverses the length of the tube, locally heating a small section of the tube. The gas stream, which moves much faster than the torch, is heated as it approaches the torched section and becomes hot enough that the precursors react with the oxygen and form oxide aerosol particles. These particles grow in size by colliding and coalescing with one another. As the gas stream leaves the torched section, it remains hot relative to the tube wall. This temperature gradient between the tube and gas stream causes the aerosol particles to move towards and deposit onto the tube wall, forming a soot layer. As the torch passes over this soot layer, the soot sinters into a solid glass layer. The process is iterated to deposit multiple glass layers on the inside wall of the tube; for example, hundreds of layers may be deposited to fabricate the core of a conventional silica fiber, but many fewer (e.g., 3-15) are typically deposited to fabricate the core of a RE-doped fiber. After layer deposition is complete, the tube is heated and collapsed to form a solid rod known as a preform. Optical fiber is drawn from the preform using standard techniques.
Not all desired dopants can be delivered using this standard MCVD method because appropriate HVP precursors do not exist. An appropriate precursor needs to have a relatively high vapor pressure (above ˜1 torr at room temperature) and should not contain any chemicals (e.g., hydrogen, water) that will adversely affect the properties of the deposited glass. Some desirable dopants such as RE elements [particularly erbium (Er) and ytterbium (Yb)] have low vapor pressure (LVP) precursors and cannot be delivered using conventional vapor-phase approaches, such as standard MCVD.
In an attempt to overcome this problem, several fabrication methods have been developed in the prior art for using LVP RE precursors, but they all have significant limitations. Below are brief descriptions of the most common methods followed by their most significant limitations.
Solution Doping: A soot layer is deposited on the inside wall of a substrate tube using standard dopants, but the soot layer is not sintered. The soot layer is soaked with a liquid solution containing a LVP RE precursor. When the solution is drained out of the tube, the soot layer acts like a sponge holding a certain amount of the solution in the pores of the soot. The soot layer containing the LVP precursor is dried, the LVP precursor is oxidized, and the soot layer is sintered. Solution doping has the following limitations: (i) The reproducibility of the LVP dopant concentration is poor from preform to preform; (ii) The amount of liquid held by the soot layer is dependent on its porosity, which is very difficult to control; and (iii) The process is very time consuming typically taking 1-2 hours per deposited glass layer compared to roughly 10 minutes for a standard MCVD glass layer.
Chelates: Preforms have been fabricated using chelate precursors of RE elements. Chelates are large organo-metallic compounds that have relatively high vapor pressures. Chelate processes have the following limitation: The chelates contain undesirable elements such as hydrogen and to date have not produced glass with the purity needed for high quality optical fibers.
Vapor Delivery: An ampoule containing the desired LVP RE precursor is placed inside an MCVD substrate tube. The ampoule is heated with an external source (such as a flame or furnace) to a temperature high enough to obtain the necessary vapor pressure. See U.S. Pat. No. 4,666,247 granted to J. B. MacChesney et al. on May 19, 1987, which is incorporated herein by reference. The ampoule approach has the following limitations: (i) The precursor vapor pressure (and therefore the amount delivered) is very sensitive to temperature, which is difficult to control because an external heat source heats the RE source material, which sits within several concentric glass tubes; (ii) Flowing gas streams cool the tubes, causing uncertainty in the actual source temperature; (iii) Only a small surface area exists between the precursor and the carrier gas, so it is unlikely that the precursor will become saturated in the carrier gas. This leads to variations in the amount of precursor delivered if the surface changes either in size or gets coated by, for example, a RE oxide layer; and (iv) When the precursor leaves the ampoule, some or all of the precursor reacts with the surrounding oxygen to form metal oxide aerosol particles. The size of these aerosol particles is not controlled. If large particles are formed, current wisdom suggests that they will form clusters, which may be detrimental for RE-doped preforms and fibers.
Liquid Aerosol Delivery: A solution, which contains organo-metallic precursors for all the desired dopants in an organic solvent, is nebulized into a liquid aerosol (droplets) and delivered into the substrate tube. As the liquid aerosol approaches the hot zone, the solvent evaporates and the resulting solid aerosol particles oxidize and deposit on the substrate tube as a layer of soot. Care is taken not to sinter the soot layer. The soot layer goes through several processing (purification) steps to remove undesirable elements such as hydrogen (H) and carbon (C). Finally the soot layer is sintered. See, for example, two papers by T. F. Morse, et al., J. Non-Crystal. Solids, Vol. 129, pp. 93-100 (1991) and J. Aerosol Sci., Vol. 22, No. 5, pp. 657-666 (1991), both of which are incorporated herein by reference. According to Morse et al., J. Non-Crystal. Solids, supra at page 96, column 2, the “solution should contain all components of the glass structure in each aerosol particle to assure microscopic homogeneity that will minimize crystalline inclusions in the glass” (emphasis added). The phrase “all components” includes, for example, the precursors of standard HVP dopants such as Ge and/or P as well as selected LVP RE elements such as Nd and/or Er. This liquid aerosol approach has the following limitations: (i) The solution is flammable and can lead to explosions; (ii) A new solution needs to be made whenever a different glass composition is desired; (iii) The process is time consuming; and (iv) The process is not expected to produce very low loss glass inasmuch as the reaction products include contaminants such as H2O and H2 as well as the desired glass oxides.
Thus, it would be desirable to have an aerosol delivery system that is capable of handling LVP precursors, such as RE precursors, without one or more of the drawbacks of the above-described liquid aerosol delivery system.
In addition, it would be desirable to have such a delivery system that is applicable to not only MCVD systems, but also other optical fiber fabrication systems as well (e.g., OVD and VAD).
Finally, it would be desirable to have such a delivery system that is applicable to the fabrication of glass bodies in general including not only optical fibers and planar waveguides but also glass soot.